The basic carrier for reed diffusers is a combination of DPG and ethanol. You can vary the proportions according to how fast you want the material to evaporate from the reeds. Some isopropyl myristate is often also included and sometimes other things, but a basic mix of 7 parts DPG to 3 parts ethanol is a good place to start.
Speaking of reeds they don't last forever: they clog, particularly if your fragrance contains some of the larger molecules; heavier, stickier oils, including many base notes.
You'll also find you need quite a lot of fragrance material - essential oils, aroma chemicals etc - to get a good effect. Especially if you use a lot of natural ingredients. You will find you need to put in as much as 30% fragrance: far more than you would in a typical EdT, so they don't work out cheap. On the other hand the one on the market that are cheap smell pretty awful, which rather defeats the point.
I'm about to launch a new range of reed diffuser fragrances myself so I've been doing all the research for some months now. One of the things that seems to be missed by most of the sellers / makers of these is how short-lived the reeds are - I'm planning on offering bundles of reeds for sale so that customers can easily replace them when they get clogged.
Hope that helps